In both the National Television System Committee (NSTC] and Phase Alternating Line (PAL) standard video systems, video data may be transmitted as a single composite signal carrying both brightness and color information. Specifically, a composite video signal includes synchronization information, a luminance (Y) component and a chrominance or “chroma” (C) component. The C component is generated by modulating U and V color components with a color subcarrier of a given color subcarrier frequency, in which the U component is the difference between the Y component and the blue color content and the V component is difference between the Y component and the red color content.
In both the NTSC and PAL systems, frames of composite video data are transmitted as two interleaved fields of lines of display pixels. Generally, each line of pixels is transmitted as a horizontal synchronization signal, a color burst signal, and the active composite video Y and C components. The horizontal synchronization signal indicates the start of the transmission of the line and gates the following color burst signal into the color synchronization circuitry. As discussed below, the color burst signal controls the recovery of the U and V color components from the received C component during demodulation. Generally, the color burst is a sample of video data represented by the subcarrier signal, which has been modulated such that the V component has a zero (0) value and the U component has a non-zero value.
A typical composite video decoder in the receiving system includes a phase locked loop, which generates cosine and sine signals at the color subcarrier frequency for demodulating the C component of the composite video signal. Specifically, the cosine and sine signals are locked in phase and frequency to the color subcarrier frequency of color burst signal of each received line. The cosine and sine signals are locked in frequency and ninety degrees (90°) out-of-phase with respects to each other. Generally, the cosine signal demodulates the C component of the composite video signal to recover the V color component and the sine signal demodulates the C component of the composite video signal to recover the U color component, following separation of the Y and C components.
Even though the color burst signal is locked in phase within three (3) degrees of phase of the locally generated sine signal, the remaining three (3) degrees of phase error still introduces anomalies in the ultimate video display. For example, in NTSC systems, phase error between the color burst signal and the locally generated sine signal, and hence the associated cosine signal as well, can cause a constant color shift in the video display. In PAL systems, this phase error can result in unwanted line-to-line patterns known as “Hanover Bars” in the video display.
In the past, video systems processing composite video signals have generally addressed the problem of phase error between the color burst signal and the locally generated cosine and sine signals by averaging equal contributions of chrominance data for even and odd display lines. These existing techniques, however, are only applicable for processing constant color display regions and desaturate the U and V components.
Hence, in order to minimize anomalies in the video display systems operating on composite video, new techniques are required for addressing the problem of phase error between a received color burst signal and the local cosine and sine signals required for color component recovery. These techniques should be applicable to both NTSC and PAL standard systems, although not necessarily limited thereto.